We report optical frequency comb generation by a continuous-wave pumped optical parametric oscillator (OPO) without any active modulation. The OPO is configured as singly resonant with an additional nonlinear crystal (periodically poled MgO:LiNbO3) placed inside the OPO for phase mismatched second harmonic generation (SHG) of the resonating signal beam. The phase mismatched SHG causes cascading χ(2) nonlinearities, which can substantially increase the effective χ(3) nonlinearity in MgO:LiNbO3, leading to spectral broadening of the OPO signal beam via self-phase modulation. The OPO generates a stable 4 THz wide (-30 dB) frequency comb centered at 1.56 μm.
We demonstrate that it is possible to obtain a mid-infrared optical frequency comb (OFC) experimentally by using a continuous-wave-pumped optical parametric oscillator (OPO). The comb is generated without any active modulation. It is based on cascading quadratic nonlinearities that arise from intra-cavity phase mismatched second harmonic generation of the signal wave that resonates in the OPO. The generated OFC is transferred from the signal wavelength (near-infrared) to the idler wavelength (mid-infrared) by intracavity difference frequency generation between the OPO pump wave and the signal comb. We have produced a mid-infrared frequency comb which is tunable from 3.0 to 3.4 µm with an average output power of up to 3.1 W.
Much progress, both experimentally and theoretically, has recently been made towards optical frequency comb generation from continuously pumped second-order nonlinear systems. Here, we present observations towards finding an integrated solution for such a system, using a titanium indiffused lithium niobate waveguide resonator. These results are compared to recently developed theory for equivalent systems. The system is seen to exhibit strong instabilities, which require further investigation in order to fully determine the suitability of this platform for stable optical frequency comb generation. 1 arXiv:1712.07448v4 [physics.optics] 1 Oct 2018Optical frequency combs (OFC's) are highly broadband coherent light sources that have found uses in a range of applications such as time and frequency metrology and molecular spectroscopy [1,2]. The usefulness of OFC's has led to rapid commercialization of the technology. The most common and reliable method of OFC generation has been through the use of mode-locked lasers, which provide high stability over long time time scales and can span large frequency ranges [3][4][5]. However, these systems tend to be quite large, and therefore there has been a strong drive towards miniaturization of the technology.Recently, most of the work regarding this goal has been directed towards systems involving the third-order nonlinearity. For example, OFC's have been produced in optical fiber cavities [6], whispering gallery mode resonators [7] and a number of microresonator systems [8,9].These are all promising systems, but they are also often plagued by extraneous effects such as mode competition and photothermal instabilities due to the high intensities required for comb generation (typically achieved through the use of strong pulsed light) [8]. A deeper understanding of these processes and possible methods for mitigating them are active areas of research [10][11][12][13].More recently, OFC's have also been observed from continuous-wave (CW) pumped second-order nonlinear systems. These systems typically offer nonlinear interaction strengths that are orders of magnitude larger than those found in third-order nonlinear systems. This may provide one avenue for reducing the amount of intra-cavity power required for comb generation, thereby providing some method of reducing unwanted thermal effects and thermallydriven mode competition. In addition, CW lasers are often simpler to work with than their pulsed counterpart. OFC generation in such systems typically occurs due to a number of cascaded nonlinearities between the various frequency components; second-harmonic generation (SHG) and optical parametric oscillation (OPO) [14]. The general scheme is shown in Fig. 1. CW pumped OFC generation utilising the second-order nonlinearity has been observed in various free-space cavity systems [14][15][16][17][18]. The recent results from CW pumped resonator systems utilising the second-order nonlinearity for comb generation has led to more complete theories that can explain much of the behavior seen f...
We present an experimental study of optical frequency comb generation based on cascaded quadratic nonlinearities inside a continuous-wave-pumped optical parametric oscillator. We demonstrate comb states which produce narrow-linewidth intermode beat note signals, and we verify the mode spacing uniformity of the comb at the Hz level. We also show that spectral quality of the comb can be improved by modulating the parametric gain at a frequency that corresponds to the comb mode spacing. We have reached a high average output power of over 4 W in the near-infrared region, at ~2 μm.
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